1. Field of the Invention
The present invention generally relates to solid state device manufacturing and, more particularly, to an interferometric optical emission detection system used for trace constituent contamination monitoring and endpoint detection.
2. Description of the Prior Art
Sodium (Na) and other alkali metals are unwanted impurities which adversely affect the operation of transistors and other solid state devices. Sodium is a mobile ion and will cause the gate of a transistor to turn on and off at different voltages. Variable operation of the gate will, in turn, cause inconsistent operation of other devices such as memory chips and the like. Sodium contamination of transistors can result from human contact with chemicals and tools used in device manufacturing (i.e., sodium ions can be transported from a person's body onto a processing tool simply by the person touching the tool), impurities present in processing reagents such as photoresist, from the manufacture and storage of these processing reagents, by the sodium which is naturally present in the atmosphere corrupting the processing plant environment (i.e., fog, mist, and rainwater seepage can carry sodium ions from the atmosphere into a processing plant), or by other means. Field Effect Transistors (FETs) are four orders of magnitude more sensitive to alkali metals than bipolars; therefore, sodium contamination is a particular concern in FET processing.
Sodium emission, as well as other trace constituent emissions, can be detected optically. Most of today's optical emission detecting systems used in transistor processing can be categorized as either monochromators with photodiode array detectors or as scanning monochromators with a photomultiplier detector. These types of optical emission systems are available from the following companies: Tracor-Northern, Plasma-Therm Analytical, Xenix, and EG&G PAR. One problem with these types of optical emission systems is that they offer low resolution detection; often greater than 1 nm and sometimes as poor as 10-20 nm. Low resolution detection results in insufficient wavelength dispersion and convolution of the desired emission signal with unwanted background and other emissive interferences. Because of this, low resolution detection also provides low sensitivity for trace species detection against the intense background emission of plasma processing tools. This is further compounded by low light throughput which is typical of conventional monochromators.
As a result of the low sensitivity of conventional optical emission systems, the usual method used for detecting sodium impurities in FETs is by electrical measurement at the end of the chip fabrication process. Clearly, it would be advantageous to have some means which is sensitive enough to detect trace levels of an impurity such as sodium ion before a large number of chips have been damaged. Ideally, potential processing problems could be identified when unacceptable levels of sodium are detected during the fabrication of FETs, and the processing could be halted temporarily to clean the tools and/or check the sodium levels in the processing reagents.
Proper etching endpoint detection is also a major concern in transistor manufacturing. One prior art method of determining the endpoint of material removal is simply timing the etch process according to the rate of material removal. To use a timed etch procedure, it is necessary to empirically determine the time at which all of the desired material has been removed, but the underlying layer has not been etched significantly. Timing is not an ideal procedure for determining the proper etching endpoint because it is indirect and the consequences of improper etching are significant. Underetching will cause a degradation of the gain of the transistor, while overetching will result in a degraded contact between the intrinsic and extrinsic base regions.
Optical emission detection has been proposed as a means for detecting the endpoint at which an etching operation should be halted. A monochromator or bandpass filter can be used to select wavelengths of light at which a desired optical emission will occur. High spectral resolution is always preferable to low spectral resolution for monitoring and analysis of plasma species because, with sufficiently high resolution, it is possible to minimize interference from other emitting plasma species. Moreover, if light throughput is held constant, peak height will increase relative to the background as spectral resolution is improved. Accordingly, high spectral resolution can provide considerable advantages in sensitivity for etch endpoint detection and in the identification of weakly emitting plasma species such as in sodium ion contaminant analysis. Unfortunately, the usual methods for obtaining high spectral resolution, e.g., narrowing monochromator slits or utilizing larger monochromators, have practical limits for use in transistor manufacturing applications. When the slits of a monochromator are narrowed, higher resolution is achieved; however, the light throughput is significantly reduced and, thus, the overall sensitivity is reduced. Using larger monochromators is unacceptable because they would require costly clean room space and are generally impractical for manufacturing applications.
Laser-induced fluorescence (LIF) is a newer technique used in transistor manufacturing for identifying trench etch endpoints and can be used to identify trace constituents. For example, U.S. Pat. 4,675,072 to Bennett et al. discloses an LIF system used to detect and control the reactive ion etch (RIE) through of a given layer in a wafer by detecting a large change in the concentration of a selected minor species from the wafer in the etching plasma. Although LIF is generally used for monitoring major species, LIF may be used to detect trace Na ion contaminants during etching; however, Na only fluoresces at the excitation wavelength of 588.996 nm or 589.593 nm and since scattered laser light is a severe limitation on the sensitivity of LIF, the presence of Na may not be easily detected because of unavoidable interference from scattered laser light off the tool walls and windows. Furthermore, methods to reduce scattered laser light, such as the use of Brewster angle windows and light baffles, are impractical for use on commercial etch tools. LIF may also be used to detect trace copper atoms which appear in aluminized lines as a means to detect an etch endpoint. However, copper atoms will exhibit fluorescence at 324.775 nm which is a wavelength that would require frequency doubling of the dye laser; a process generally regarded as infeasible for manufacturing applications because more powerful lasers are required for this task and a skilled laser operator would be required.
Several laser interferometer techniques are now in standard practice for optically detecting line widths and etch endpoints. U.S. Pat. No. 4,454,001 to Sternheim et al. and U.S. Pat. No. 4,680,084 to Heimann et al. are directed to etch monitoring using laser interferometric methods whereby the thickness of the region being etched is simultaneously monitored. U.S. Pat. No. 4,838,694 to Betz et al. discloses a laser interferometry process which uses the reflected laser beam. U.S. Pat. No. 4,717,446 to Nagy et al. discloses a method, using a monitor wafer which is correlated to the endpoints of the working wafer, for detecting the endpoint of the etch of an epitaxially grown silicon whereby a laser is used to measure the etch rate of the monitor wafer by measuring the reflected light off the oxide layer. U.S. Pat. No. 4,602,981 to Chen et al. is directed an impedance monitoring technique for plasma etching wherein endpoints are detected by an impedance change of the plasma; however, the Chen et al. reference does disclose that laser interferometry is well established in the art and points out that the laser measures the thickness of the film removed as the etch process proceeds. U.S. Pat. No. 4,758,304 to McNeil et al. discloses an apparatus for ion etching which utilizes an interferometer for surface monitoring. The methods described in Chen et al. and McNeil et al. are generally insensitive to the precise wavelength of the detected light. Before the invention thereof by the applicants, interferometers have not been used in solid state device manufacturing for detecting trace emitting species in a complex background spectra.
Furthermore, several techniques for controlling interferometers are now in common practice. U.S. Pat. No. 4,482,248 to Papuchon et al. discloses an interferometer used for optical filtering. U.S. Pat. No. 4,711,573 to Wijntjes et al. discloses a dynamic mirror alignment control which utilizes an interferogram for analyzing sample materials wherein a closed loop servo motor is used to maintain position orientation. U.S. Pat. No. 4,448,486 to Evans discloses the use of mirrors in a Fabry-Perot interferometer used to change the bandwidth of the Fabry-Perot interferometer. The applicants invention describes completely new methods for using and controlling interferometers in transistor manufacturing.